In the serial type ink jet recording apparatus, a multi-path recording method is generally used to reduce connection stripes appearing a boundary portion between adjacent recording scans and a density unevenness resulting from a variation in ink ejection properties of respective nozzles. With such a multi-path recording, it is known that the evenness of the output images is improved with increase of the number of the multi-paths (the number of recording scans for a unit area), whereas on the other hand, the throughput decreases therewith. Therefore, a recording method with which high image quality images are outputted with as small number of multi-paths as possible is desired.
A method with which the throughput is improved, there is a bi-directional multi-path recording in which the recording operation is carried out both in a forward path and a backward path of a carriage carrying the recording head. Using the bi-directional multi-path recording, the number of recording scans is approximately one half that in the unidirectional recording with the same number of the multi-paths, and therefore, the time required for the recording operation can be reduced. However, it has been confirmed that the bi-directional multi-path recording may cause another unevenness between bands due to a variation in a density, a hue or a glossiness occurring at intervals corresponding to a feeding distance of the recording material. In the present Specification, the new unevenness between adjacent bands occurring in the bi-directional multi-path recording is called inter-band unevenness.
Parts (a) and (b) of FIG. 1 are schematic views illustrating specifically a cause and a phenomenon of the inter-band unevenness. Part (a) of FIG. 1 illustrates the case of four path bi-directional recording using a recording head 1000 including four arrays of nozzle groups for ejecting cyan ink (C), magenta ink (M), yellow ink (Y) and black ink (K), respectively. The recording head 1000 carries out the forward path recording scan and the backward path recording scan alternately in a direction of X, during which between the recording scans, the recording head 1000 moves relative to the recording material in a direction of Y by the distance corresponding to a width of a unit area each time.
Here, a left-hand end portion of a first band having a unit area width, first. At this time, the area receives the inks in the order of C-M-Y-K in the first recording scan, and then after a relatively long time corresponding substantially to one reciprocal scanning operation of the recording head, it receives the ink in the order of K-Y-M-C. In a left-hand end portion of a second band adjacent to the first band, the area receives the inks in the order of K-Y-M-C, and then after a relatively short time corresponding substantially to a reverting operation of the recording head, it receives the ink in the order of C-M-Y-K. Thereafter, in the third band adjacent to the second band and odd number bands following the third band, the inks are applied in the same conditions as in the first band, and in the fourth band adjacent to the third band and even number bands following the fourth band, the inks are applied in the same conditions as in the second band. Thus, two kinds of bands having different order and intervals of the color ink applications to the recording material appear alternately in the Y direction. The different order and different intervals of the color ink applications influence quite significantly the density, the hue and/or the glossiness of the image formed on the recording material. As a result, the inter-band unevenness are remarkable in the image comprising the bands shown in part (b) of FIG. 1, which are different in the application order and/or timing.
In order to reduce the image defect of such an inter-band unevenness in the multi-path recording, it is effective to adjust a recording permission ratio of each nozzle of the recording head.
Here, the recording permission ratio is a ratio of the pixels for which the recording is permitted in one recording scan (movement) of the recording head, to a plurality of pixels included in a predetermined area. For example, a recording permission ratio of a nozzle is a ratio of the pixels in one nozzle passing by one movement (scanning) of the recording head, for which pixels the recording by the nozzle is permitted, to all the pixels of such a nozzle. The permission and non-permission of the recording for each pixel are ordinarily determined by a mask pattern prepared beforehand. Therefore, the recording permission ratio of each nozzle is determined by the mask pattern, and in the present Specification, the adjustment of the recording permission ratio means adjustment of an arrangement of the recording permission pixels in the mask pattern.
For example, Japanese Laid-open Patent Application 2000-108322 and Japanese Laid-open Patent Application 2002-96455 specifically disclose a structure in which by using the improved mask pattern so as to determine the recording permission ratios of the nozzles with a predetermined bias, connection stripes and density unevenness are less conspicuous even with a small number of multi-paths, is disclosed.
In the multi-path recording, in order to maintain an evenness of the image, it is effective to control the recording permission ratio in respective recording scans (nozzle block) by a method other than those disclosed in the Japanese publications. The appearing of the inter-band unevenness is different depending on various conditions such as the kind of the recording material, the kind of the ink, but the evenness of the image can be maintained if the recording permission ratio can be adjusted using a mask pattern in accordance with various conditions.